A jock is absolutely a utilitarian device. I'm sure that who ever invented it would have been a member of LPSG if it were around back then. If you got a lot of swinging meat, then for certain activities, it is really necessary. I can't imagine anyone feeling as though they were 'caught' in their jock strap. It's no different than being 'caught' in your underwear. And if it's guys at the gym giving you a hard time, just point out that some people will just never need a jockstrap because their stuff doesn't swing.
As far as it being a fetish... I really like them and they turn me on, but they only work on certain guys... I can't wear one, because for one, I don't really fill out the front, and for two (the bigger reason), I just don't have an ass... a jockstrap just highlights my skinny butt.